This invention relates to a movable electrode structure of a resistance welding apparatus for nut with which a nut is welded on a workpiece.
Japanese Patent Application Laid-Open Gazette No. 6-238461 discloses an example of a resistance welding apparatus for nut. This apparatus includes a fixed electrode, a movable electrode placed above the fixed electrode, and a nut feeding means for feeding a nut to a position between the fixed and movable electrodes. The movable electrode is provided with a guide pin which protrudes downward so as to be inserted into a hole of a nut. The nut feeding means is for holding a nut on a nut seat provided on the front end of a rod and feeding the nut to the position between the fixed and movable electrodes. The nut seat is supported to the tip of the rod so as to be swingable between a position that a seat face for holding the nut is substantially horizontal and a position that the nut seat is dangled, and is urged by an urging means so that the seat face is substantially horizontal.
In the above resistance welding apparatus, when a nut is fed below the movable electrode, the movable electrode moves down to insert the guide pin into the hole of the nut, forcedly dangle the nut seat and then position the nut on a portion to be welded of the workpiece overlying the fixed electrode. Since the nut is only fitted on the guide pin when the movable electrode receives the nut from the nut feeding means and moves down, the movable electrode is set at a falling speed greater than the speed of free fall of the nut so that the nut cannot fall out of the guide pin.
The guide pin is urged in a direction of protruding from the body of the movable electrode by a spring. Thus, the guide pin contacts a positioning pin of the fixed electrode to retract in the movable electrode while the nut and the workpiece are sandwiched between the fixed and movable electrodes and are welded, whereas the guide pin protrudes from the movable electrode by the urge of the urging means when the movable electrode moves upward after the welding.
In the case that the movable electrode has a long falling stroke after receiving the nut from the nut feeding means, however, the falling speed of the movable electrode must be set considerably great in order to fall the movable electrode at a speed faster than the speed of free fall. This makes it difficult to smoothly carry out an operation of transferring the nut from the nut feeding means to the guide pin of the movable electrode and makes it difficult to smoothly position the nut onto the workpiece overlying the fixed electrode.
In addition, though the workpiece having the welded nut must be lifted and shifted to a different position for the next welding work, the guide pin immediately protrudes when the movable electrode rises after the welding. Therefore, when the workpiece is promptly lifted and shifted to a different position according to the rise of the movable electrode, the previously welded nut may be caught on the protruding guide pin. Thus, the guide pin interferes with a prompt welding work.